From looking at the source, it looks like ZipArchive.dll is something provided by your installer. It looks like in the latest build this DLL isn't being built into the installer...

Sorry for leading you on a wild goose chase, but there is no ZipArchive.dll. There used to be, but I forgot I switched it to a static library (compiled into Sigil.exe) a long time ago. So there's nothing to look for.

Honestly I don't know what's causing this. It seems like it's a problem with the Visual C++ runtime side-by-side assemblies, but what exactly is going on, I can't tell.

Can you perhaps try building Sigil from source on your computer? If you don't get the error with the compiled exe but do with the downloaded one, then it's a packaging/installer problem on my end. If you get the error, then it's your machine.

If you don't have Visual Studio, you can get the command line compiler with the Windows Platform SDK. If you need help setting up this build system, just say so.

This application has requested the Runtime to terminate it in an unusual way.

That probably means that an unhandled exception was thrown. And the only thing that throws exceptions in Sigil is ZipArchive. It seems it couldn't read your zip file.

This could be a problem with the specific epub file, or a permission error. Calibre uses python's zipping utilities, and those can create bad zip files (I already had to create a workaround for such a problem).

Let's eliminate the epub file question. Download the epub file from this thread and try to open it in Sigil. I know for a fact this one works just fine.